I don't know what your specific use case is but I am assuming you want to give users the be able to view all the organizations on your portal and then assign themselves to those organizations.

If that is the case there is a way by adding certain permission to their roles. For example if you go to your control panel and go to "Roles" and click on the edit button on the "User" role you will see different permissions. Since the User role is a Regular role you will have a "Portal" section in the "Add Permissions" drop down menu (Organization roles and Site Roles will not have this option available).

In the "Add Permissions" drop down menu under "Portal", if you go to "Users and Organizations" and under "Organizations" you can check "Assign Members" and "View". What this does is it grants the user the ability to see the "Users and Organizations" control in their control panel. This way a user can then go into their own control panel and click "Users and Organizations" and see all the organizations and then assign themselves to that organization.

In my case, users can request membership to the Community (associated with an organization). And when the Admin approves the membership request, requester become member of the community only, and not to the assiciated Organization, which restricts the ability to have hierarchical search user search. As of now, Admin has to manually make the requester member of the organization.